In connection with the offshore petroleum industry, it is often required to position a vessel in immediate proximity to a harbour, a platform or similar, to be able to carry out loading and unloading of goods, and to load oil and gas onto large vessels etc. However, this may be extremely difficult far out at sea, where huge waves and high wind affect the movement of the vessel. Some platforms also float in such a way that both the vessel and the platform move independently of each other. In the North Sea, a series of cases has been reported, wherein vessels have hit an adjacent platform. This may lead to damage on the vessel, the platform, and the crew, and also cause leakage of oil and gas, which leads to pollution, and the danger of explosion. As a consequence of this, it is desirable to automatically position the vessel in relation to the platform, which is is done by use of a dynamic positioning system (DP-system). The method and the system according to the present invention is a reference system which provides input in such a DP-system.
Today the petroleum industry demands several independent reference systems based on different principles, as inputs to such DP systems. The DP manufacturers and their users have demanded systems with more robustness, ease of use, and integrity properties related to this type of usage.
The reference systems currently utilised for relative positioning applications comprises:                Laser based systems, e.g. “Fanbeam” (trademark) from MDL (Measurement Devices Ltd, UK).                    These systems utilise a laser to track passive light reflectors, and typically outputs the distance and bearing to each reflector. These systems utilise a scanning laser device to track the reflectors. The laser based systems have severe limitations, i.e. tracking of false targets (reflectors), reduced accuracy and loss of signal due to sunlight saturation as well as absorption and loss of signal due to snow, heavy rain or fog in the path of the laser light. Another severe limitation is low reliability due to movable parts (motor and gearbox) driving the scanning lasers. In addition heavy roll and pitch may cause the narrow laser beam to loose track of targets.                        GPS based systems.                    These systems compare measurements against the GPS satellites, and transmit the data over some form of radio link between the vessel and the structure it is positioning itself relative to, and compute the relative distance and bearing between the vessel and the target system.            The GPS based systems are quite good and are based on technology with high reliability and low maintenance cost. However, it can be vulnerable to degraded satellite constellation, specifically if the vessel is near a large structure that is blocking the view of the horizon.                        Radar based systems, e.g. “Artemis” (trademark) from CHL NETHERLANDS B.V. and “RadaScan” (trademark) from GCS (Guidance Control Systems), UK                    These systems utilise a conventional rotating radar antenna both on the vessel and at the remote station. The “Artemis” measures the distance and bearing between two stations, and the antennas are always aligned with each other. This system can only track one target, and can only be used by one vessel at a time. It is expensive, and reliability is an issue due to the engine controlled antennas. The “RadaScan” is described in Modulated Microwave Position & Heading Reference Sensor, Jan Grothusen, Dynamic Positioning Conference, Sep. 16-17, 2003). This system utilises conventional rotating radar antenna with its inherent limitations, and measures its position relative to identifiable transponders.                        
Patent application EP 0 777 133 A1 presents FM-CW radar apparatus (FMCW—Frequency Modulated Continuos Wave) for determination of the distance and the relative velocities for a number of targets in connection with anti-collision systems for automobiles. Radar technology is described in detail in “Radar Handbook”, Skolnik 1970.
U.S. Pat. No. 6,084,530 presents a radio communication system comprising an interrogator which generates a continuous radio signal. One or several tags in the system receive the radio signal, and a modulator modulates the reflection of the radio signal by use of a carrier signal and uses this to create a reflected and modulated signal. The signal is once again received by the interrogator, which based on the frequency of the carrier signal, can determine the identity of the tag. The system also is able to show vibrations or movements within the tag.